1. Field of the Invention
This invention pertains generally to gas turbine driven powerplants, and more particularly to a gas turbine driven powerplant in which a mixture of steam and methane, or another combustible fuel, is reformed or partially oxidized to produce a hydrogen-rich fuel which is used to fuel both the primary combustor and a reheat combustor. This invention also pertains to use of said fuel, or the steam/combustible feed that becomes said fuel, as turbine coolant in order to allow higher reheat temperatures to be achieved and/or to reduce the need to bleed compressed air for cooling purposes.
2. Description of the Background Art
In GB-A-2232721, published on Dec. 19, 1990, the concept of a chemically recuperated gas turbine ("CRGT") is discussed and certain CRGT designs are presented. However, while that patent discusses the advantages of developing a methane/steam reaction within the exhaust stream of the gas turbine-driven powerplant to produce a hydrogen-rich fuel which can be used to fire the powerplant with low emissions of oxides of nitrogen, it also dismisses much of the advantage as impractical based on cost, efficiency and design considerations. The patent explains that exhaust temperatures required to develop an efficient hydrogen-producing steam/methane reaction (1150.degree. F. to 1800.degree. F.) can only be developed through the use of a reheat combustor and a power turbine designed to withstand temperatures considerably higher than the current capabilities of such turbines. The patent apparently assumes that development of a reheat combustor would be costly based on the development history of flame holders and combustion cans for the primary combustors of existing gas turbine engines. Development of a suitable power turbine for such a cycle is commonly deemed problematic by turbine manufacturers. If one addresses this problem through the traditional approach of bleeding cooling air from the compressors, much or all of the efficiency advantage would likely be lost because the power turbine is larger than the high pressure turbine and requires more air for cooling and/or a lower temperature limit.
The patent goes on to explain that the complexity and expense of developing a reheat combustor and associated power turbine makes it desirable to develop a chemically recuperated gas turbine that avoids this expense and complexity. The patent then describes five embodiments which all avoid the use of a reheat combustor either by the use of a duct burner positioned downstream of the power turbine and upstream of the reformer or by postulation that super-activated catalysts may allow the methane/steam reforming reaction to occur at lower temperatures, minimizing or eliminating the need for duct-firing. However, this problem cannot be resolved by development of new catalysts. The problem is one of thermodynamics, rather than of inadequacy of the catalyst. The chemical equilibrium at lower temperatures is not favorable in that insufficient hydrogen will be produced. Today's catalysts already produce nearly equilibrium compositions, but high temperatures are still required to produce the quantities of hydrogen required to control NO.sub.x and recuperate exhaust heat. The patent goes on to recognize that duct firing reduces efficiency since fuel is consumed by the burner but notes that chemical recuperation increases cycle efficiency and, therefore, even when the burner is utilized the efficiency enhancements obtained through chemical recuperation will at least offset a portion of any efficiency reduction and the system may provide an overall increase in efficiency.
Thus, while GB-A-2232721 teaches the potential advantages of combining chemical reformers with gas turbines, it teaches away from development of reheat combustors and power turbines that can withstand the temperatures typically required to make the chemically recuperated cycle work efficiently. Further, while reheat combustion will increase power output, it also raises the temperatures throughout the power turbine thus requiring a major bleed of cooling air which significantly reduces the net efficiency gains. Therefore, there is a need for a gas turbine driven powerplant which includes a reheat combustor which does not require a flame holder, does not entail the development cost associated with turbine cooling, and which requires little or no modification of existing gas turbine engine technology. The present invention satisfies that need, as well as others, and overcomes the deficiencies in conventional reheat technology.